Normally, in a TOFMS, a certain amount of kinetic energy is imparted to an ion derived from a sample component to make the ion fly a distance within a space. The period of time required for the flight is measured, and the mass-to-charge ratio of the ion is calculated from the time of flight. Therefore, if there is a variation in the position of the ions and/or the amount of initial energy of the ions when ions are accelerated and begin their flight, the ions having the same mass-to-charge ratio will vary in their times of flight, which leads to a deterioration in the mass-resolving power or mass accuracy. One commonly known technique for solving this problem is the orthogonal acceleration TOFMS (which is also called the “perpendicular acceleration” or “orthogonal extraction” TOFMS) in which the ions to be sent into the flight space are accelerated in an orthogonal direction to the incident direction of the ion beam.
Meanwhile, in recent years, for the identification or structural analysis of substances having high molecular weights or substances having complex chemical structures, an MSn analysis (which is also called the “tandem analysis”) has been commonly used, in which an ion having a specific mass-to-charge ratio is dissociated in one or more stages by a collision induced dissociation or similar technique, and the thereby generated product ions are mass-analyzed. Commonly known types of mass spectrometers capable of an MSn analysis are as follows: a triple quadrupole mass spectrometer, which includes two quadrupole mass filters placed before and after a collision cell for dissociating ions which contains a quadrupole ion guide (or multipole ion guide with a different number of poles); an ion trap mass spectrometer, which uses an ion trap having the function of separating ions according to their mass-to-charge ratios as well as the function of performing the dissociation operation on the ions; and an ion trap time-of-flight mass spectrometer, in which the aforementioned type of ion trap is combined with a TOFMS.
A quadrupole time-of-flight mass spectrometer (which is hereinafter called the “Q-TOFMS” according to a convention), which includes a quadrupole mass filter and orthogonal acceleration TOFMS respectively placed before and after a collision cell in order to make use of the high capability of the orthogonal acceleration TOFMS, is also commonly known.
FIG. 10A is a schematic configuration diagram of a collision cell and orthogonal accelerator in a Q-TOFMS described in Patent Literature 1. FIG. 10B is a diagram showing the potential distribution on the axis C (which in the present case is the ion beam axis) in FIG. 10A. FIG. 10C is a timing chart of the voltage applied to the exit gate electrode in FIG. 10A and the orthogonal acceleration voltage.
As shown in FIG. 10A, this Q-TOFMS is provided with an ion guide 51 within the collision cell 50 which dissociates ions. This ion guide 51, in conjunction with the entrance gate electrode 52 and the exit gate electrode 53 placed before and after itself, constitutes a linear ion trap. In this example, each of the entrance and exit gate electrodes 52 and 53 doubles as the entrance end face or exit end face, respectively.
A precursor ion having a specific mass-to-charge ratio selected in a quadrupole mass filter (not shown) is dissociated in the collision cell 50, with the potentials at the entrance and exit gate electrodes 52 and 53 increased to higher levels than the potential at the ion guide 51 so as to temporarily trap the generated product ions (and precursor ions which have not been dissociated) within the inner space of the ion guide 51. At a later point in time, the voltage applied to the exit gate electrode 53 is temporarily lowered so that the ions which have been trapped until that point in time are released from the collision cell 50 at a predetermined timing. The released ions are introduced through the grid electrode 54 and the skimmer 55 into the orthogonal accelerator 56 of the orthogonal acceleration TOFMS along the X axis. When an acceleration voltage is applied to the orthogonal accelerator 56 at a predetermined timing, the ions are accelerated in the Z-axis direction and introduced into the flight space (not shown).
The solid line in FIG. 10B represents the potential distribution when ions are trapped within the inner space of the ion guide 51. In this situation, since the potential at the exit gate electrode 53 is higher than the potential at the ion guide (rod electrodes) 51, ions moving toward the exit gate electrode 53 are pushed back and trapped within the collision cell 50. The broken line in FIG. 10B represents the potential distribution when the voltage applied to the exit gate electrode 53 is lowered. In this situation, the potential slopes from the exit end of the collision cell 50 down to the orthogonal accelerator 56, whereby the ions which have been trapped until that point in time are accelerated toward the orthogonal accelerator 56.
Although the ions having various mass-to-charge ratios trapped within the inner space of the ion guide 51 are almost simultaneously released from the ion guide 51, the ions become spread in their travelling direction (i.e. in the X-axis direction) before they reach the orthogonal accelerator 56. That is to say, the ions are given approximately equal amounts of acceleration energy, which means that an ion having a lower mass-to-charge ratio travels at a higher speed. Therefore, an ion having a lower mass-to-charge ratio reaches the orthogonal accelerator 56 earlier, followed by other ions arriving at the orthogonal accelerator 56 while being delayed in ascending order of their mass-to-charge ratios.
In the orthogonal accelerator 56, an acceleration voltage (which is called the “push-pull voltage” in Patent Literature 1) is applied at a predetermined timing. Only the ions which are passing through the orthogonal accelerator 56 when the acceleration voltage is applied are accelerated toward the flight space; the other ions are wasted. The rate of use of these ions is called the “duty cycle”, which is defined as follows (for example, see Patent Literature 2):Duty Cycle [%]={(amount of ions used for the measurement)/(amount of ions which have reached the orthogonal accelerator)}×100
The dissociation of the ions within the collision cell 50 produces ions having various mass-to-charge ratios. In the Q-TOFMS described in Patent Literature 1, in order to improve the duty cycle of the ions having a mass-to-charge ratio of interest, the delay time tD from the point in time t1 where the pulsed voltage for releasing the ions from the collision cell 50 is applied to the point in time t2 where the acceleration voltage is applied in the orthogonal accelerator 56 is adjusted according to the mass-to-charge ratio of the target ion to be subjected to the measurement (see FIG. 10C). In this operation, the acceleration voltage is applied at the timing when the ion which the analysis operator is paying attention to passes through the orthogonal accelerator 56. Therefore, the duty cycle for the target ion having the specific mass-to-charge ratio is improved, so that the detection sensitivity of the ion will also be improved.
However, the previously described Q-TOFMS has the following problems.
(1) In the previously described Q-TOFMS, when the mass-to-charge ratio of the ion for which the duty cycle should be improved is changed, it is necessary to accurately regulate the delay time tD. Regulating the delay time of a pulsed signal at the levels of microseconds requires a high-precision delay line or similar element. However, such an element is expensive. Additionally, in the case where the operation of temporarily trapping the ions by the linear ion trap and performing a mass spectrometry on those ions by the TOFMS is repeated with a fixed cycle, i.e. at regular intervals of time, the control will be complex if the timing of the acceleration in the orthogonal accelerator 56 varies depending on the mass-to-charge ratio of the target ion.
(2) In the previously described Q-TOFMS, the duty cycle for ions other than the ion which the analysis operator is paying attention to becomes low (or those ions are practically almost undetectable). As in the MRM (multiple reaction ion monitoring) or precursor ion scan measurement, if the mass-to-charge ratio of the product ion to be monitored is fixed, the previously described Q-TOFMS is useful since only that specific product ion needs to be detected with a high level of sensitivity. However, the device does not allow the duty cycle to be simultaneously improved for a wide range of mass-to-charge ratios of the ions. Therefore, for example, as in the case of a product ion scan measurement or normal scan measurement which includes no fragmentation of the ion, if a mass spectrum covering a wide range of mass-to-charge ratios needs to be obtained, it is necessary to repeat the measurement a plurality of times with the mass-to-charge ratio range shifted each time.
A solution to the previously described problem (2) is a TOFMS described in Patent Literature 3. In this TOFMS, an ion guide having the function of trapping ions is axially divided into three segments so that a different voltage can be applied to each segment of the ion guide. Regulating the radio-frequency voltages applied to those segments of the ion guide changes the thereby created pseudo potential, making it possible to control the behavior of the trapped ions in each of the axial and radial directions. Accordingly, by appropriately changing the radio-frequency voltages according to the mass-to-charge ratio of the ion to be released, it is possible to make ions having different mass-to-charge ratios be individually released in a desired order and almost simultaneously arrive at a specific point in space.
However, this TOFMS requires the ion guide to be axially divided and additionally equipped with a power source capable of applying a different radio-frequency voltage to each segment of the ion guide. Furthermore, the sequence for changing the voltage according to the mass-to-charge ratio is complex.
Those problems are not unique to the Q-TOFMS; an ion trap time-of-flight mass spectrometer, in which the ions temporarily captured within a three-dimensional quadrupole ion trap are collectively ejected from the ion trap and mass-analyzed, has similar problems to those which occur in the previously described type of orthogonal acceleration TOFMS. In this type of mass spectrometers, if ions are spread in their travelling direction before they arrive at the ion injection hole of the ion trap, only the ions which arrive at the ion trap within a predetermined time range can be captured within the ion trap; the other ions are repelled at the ion injection hole or directly pass through the ion trap, without being used for the measurement. Therefore, if ions arrive at the ion injection hole of the ion trap in a temporally shifted form according to their mass-to-charge ratios, only the ions within a limited range of mass-to-charge ratios can be captured by the ion trap, so that it is impossible to perform the measurement for a wide range of mass-to-charge ratios of the ions with a high level of sensitivity.